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Aviation History
1957
1957 - 1716.PDF
806 GNAT M k I . . FLIGHT^ 22 November 1957 11 and 12; and the latter frame passes shear loads across them. It also holds the front wing attachments and the forward mounting of the heavy, built-up beam which supports the front of the gun and its recoil loads on each side. Frame 12 further separates Nos. 1 and 2 tanks. Between frames 12 and 16 the main fuselage loads are taken bythe magnesium shear walls which here form the inner wall of the gun bay. The two Aden guns are mounted almost on their backsand supported by the beams from frame 12 and on the two forg- ings on frame 16. The outer skin is here entirely unstressed andforms a panel which uncovers the entire gun bay. Immediately beneath the guns are the ammunition cans, each of which holds115 rounds of 30 mm ammunition, disposed athwartships. Each ammunition belt therefore emerges over the forward lip of its can,turns down and runs through a flat duct underneath the can before turning sideways and upwards into the breach. Expendedcases are thrown through tubes and emerge high up in the sides of the fuselage so as to pass over the top of the tailplane. Thesetubes have recently been raised since it was found that some cases were striking the tailplane. Expended links are passed down chutesoutside the can, across the fuselage and into the space being cleared in the opposite can as fresh rounds feed to the gun. Acanvas separator prevents confusion. Earlier, links were collected in the fuselage, but the aileron-drooping linkage has since cutacross the available space here. Frame 16 carries the main wing pick-ups, the main under-carriage, the rear mountings of the guns, and the aileron actuators and drooping gear; and its reinforced web locates the intake ductand forms the front wall of Nos. 3 and 4 tank bays. The first four of these tasks are performed on each side by large die forgings,bolted to the frame web out-board of the shear wall. They can truly be said to earn their keep in the best Gnat manner. Frame 18 divides Nos. 3 and 4 tank bays from the engine baywhich is flanked by the shear walls, mainwheel wells and Nos. 5 and 6 tanks. The underside is a stressed panel giving access tocontrol runs and engine accessories. Frame 19 is sloped back past the wheel wells and joins frame 20. Castings bolted to the shearwall just forward of frame 19 provide attachment for the main- wheel jacks and radius rods, the shear walls being dished here toaccommodate the wheels. Light-alloy sheet webs close off the fairings at frame 22A, whileframe 22B, which is clamped against it, carries a steel fire wall and is designed to redistribute the shear stresses from the shear wallin the forward fuselage into the outer skin of the tail section. As stated earlier, fuselage loads are carried across the break between frames 22A and 22B by eight spigots which are bolted into fittings. Special provision is made for achieving a good fit without having to observe too-close jigging tolerances. From this point aft, the fuselage becomes a conventional monococque, with Z-section stringers and light frames. Frame 26 is inclined rearwards and is integral with the forwardspar of the fin. In similar fashion, frame 27 picks up the rear fin spar; and the tailplane pivots are located in boxes formed by bothvertical and horizontal webs between the frames. The pivots themselves are magnesium castings in which the ball-bearinghousings are slit and clamped by bolts to allow for differential expansion. Both fin and horizontal tail units are of conven-tional built-up construction, all moving surfaces having inset hinges and lead mass balances. - , _, GNAT SYSTEMS 'T'HE simplicity of the Gnat flying control system is in keeping-•- with Petter design traditions, with the undercarriage provid- ing airbrake surfaces and also operating the flap-drooping system.In fact, a complete range of services is provided with only six hydraulic actuators, three for the undercarriage, one for eachaileron and a twin unit for the horizontal tail. The rudder is directly mechanically operated through a cabletransmission driven by a short push-rod from the standard S.B.A.C. rudder pedals and ending with a push-rod linkage insidethe fin. The pedals are adjustable for reach. For control on the ground, individual toe pedals directly operate each mainwheeldisc brake hydraulically. Neither Maxaret nor power assistance is considered necessary. The nosewheel is self-centering, and isnot steerable. In order to reduce rudder cable travel, and thereby the required size of the quadrant working the push-rods in the fin,double pulley units providing a 2:1 reduction are mounted just forward of this point. Spring centering with a positive indicationof central position is provided for comfort and to allow "feet off" flying. It is foreseen that a viscous yaw-damper cylinder will beinterposed in the linkage in the fin. For aileron operation the control column is articulated just below the hand grip, the pivot carrying a pulley wheel which The fuselage, looking for- ward from frame 22A. The flanks are the magnesium shear walls and the end of the intake duct is at frame 18. Accessories ere dis- tributed near the stressed panel in the under-fuselage. engages the cable running down to the base of the column andthence rearwards to a further pulley lying flat between tail- plane and rudder quadrants. This pulley in turn carries a cablelinkage to two sprockets engaging a section of chain in the main aileron cable loop. The valves of the Lockheed aileron Servo-dynes are controlled by push-rods operated from the centre of a beam? one end of which is moved from the butterfly segmentscarrying the main aileron cable loop. The other end of the beam is held by a link mounted, to port, on a wheel directlygeared to the main undercarriage pivot-pin. Thus the rotation of the undercarriage leg alters the datum on which the aileronoperating rod moves. The link is so mounted on the geared wheel that the rotation of the undercarriage from the "up"to the "airbrake" position imparts a negligible droop to the aileron—something less than \ deg. A similar datum-varyingarrangement is incorporated on the starboard side and operated by a cable linkage from the port side. Formerly each main-wheel leg drove its own aileron-drooping linkage, but it was found that, during lowering, one leg was slightly ahead of theother and a momentary asymmetric droop resulted. In order to reduce aileron response near the neutral position,the three pulleys in the aileron circuit are oval. Feel is pro- vided by a plain spring in the control column. In case of failureof the hydraulic supply to the aileron Servodynes the existing linkage between stick and valves has been made strong enoughto accept the full mechanical loads for manual operation. Tru Servodyne body, carrying the valve, operates the aileron; andin manual control a slight backlash due to valve movement occurs. It is, however, very slight and allows automatic andimmediate manual reversion. Total undercarriage-leg travel is 125 deg; and the airbrakeposition is reached after 20 deg of main leg and 50 deg of nose leg travel. The normal range of aileron movement is —14 degto +12.5 deg but, with the undercarriage fully down, the ailerons are drooped 22 deg and their travel is increased from —21.5to +18 deg. The Servodynes themselves accommodate these travels with a total movement of only 3.2in. The latest Gnat horizontal tail control retains an elevatormounted on a moving tailplane, but for all normal flying the elevator is locked to the tailplane by a bolt in the horn balancethe two then acting as a one-piece slab unit. Normal operation is as follows: Fore and aft movement of the stick operates abutterfly segment carrying the cable loop running in the under- fuselage to a quadrant on the rear-fuselage floor. This quadrantcarries a specially cut cam which imparts non-linear response to a push-rod working the hydraulic valve of the Hobson unit.The cam has been so profiled that tail response to stick move- ment is reduced near the neutral position. Thus, at very highspeeds when small angular movements are required, low tail- plane response makes the system less sensitive. Response isgreatly increased over the higher stick deflections. As is well known, the Hobson unit consists of a hydraulicmotor, controlled from the cockpit, driving twin screw-jacks which rotate the tailplane pivot through an epicyclic reductiongear in which the motor shaft drives the sun wheel and the planet wheels drive the jacks. Trim is provided by an electricmotor controlled from a switch on the control-column grip, arranged to drive the annulus of the epicyclic gear. The trimmotor can thus move the tailplane through its full range of — lfrj- deg to + 2| deg without affecting the stick datum. The tailplane quadrant in the rear fuselage can also operate apush-rod linkage to each elevator, but for all normal flying the elevators are locked by bolts to the tailplane. The push-rods aretelescopic in this condition and the piston section of each moves two springs housed on the cylinder section to provide spring fcelfor the horizontal tail system. The springs on each side are pre- loaded to give a stick load over the central 15 deg of stick move-ment twice that over the rest of the range. Centering cams at tne base of the stick positively show the neutral position. Peak push-force is about 25 lb and the peak pull force slightly higher. This varying stick rate causes a relatively high force over the small
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